The present invention relates to a device with a circulating optical delay line of the type using an optical fibre as the delay element.
Devices of this type are used with particular advantage in the field of high resolution radars, particularly pulse radars of the M.T.I. (moving target indicator) type, and particularly of the non-coherent type which cancels out the fixed echos with the aid of a delay line.
It has also been demonstrated that the order of magnitude of the "pass band--delay" product was such that only delay lines produced with the aid of optical fibres would be suitable. In addition, optical fibres also have low transmission losses.
An optical fibre delay line device for a radar of the type referred to hereinbefore is especially described in the article by CHANG et al: "Non-coherent radar moving target indicator using fiber optic delay lines" published in the "IEEE Transactions on circuits and systems", volume CAS-26, no. 12, December 1979, pp. 1132-1134.
Devices of this type comprise a source emitting light pulses, optically coupled to one of the ends of an optical fibre for injecting these pulses there. They are then propagated in the guided mode into the optical fibre core to the other end and with a finite speed. Thus, the transit time is directly dependent on the length of the optical fibre and the speed of light in the material of the optical fibre core.
The other end of the optical fibre is coupled to optoelectronic detection means. A prior art device of this type is described in U.S. Pat. No. 4,166,212. In the case of the radar, the delay supplied by the delay line is equal to the time interval separating two pulses emitted by it. Typically, the orders of magnitude are in the respective ranges of a nanosecond for the width of the pulses and a millisecond for the time interval separating two pulses.
In view of the transmission speed of light in an optical fibre, it is necessary to use an optical fibre with a length of several kilometers, which implies an optical fibre with very low losses. This type of optical fibre is naturally onerous. Moreover, although the optical fibre is normally wound on to a coil former, the assembly is relatively cumbersome.
Finally, for certain applications in which the optical fibre length is particularly great, it can be necessary to use regenerative repeaters inserted in the line, which increases costs and complexity.
The invention is therefore directed at a device with a circulating optical delay line, in which the necessary optical fibre length is divided by a factor of 2. For this purpose, use is made of the properties of certain semiconductor laser diodes of also being photodetective under certain conditions. These properties are e.g. described in the article by ALPING et al, "Detection at G bit/rates with TJS GaAlAs laser" which appeared in the journal "Optics Communications," vol. 44, no. 6, 15.2.1983, pp. 381-383.